In the vast majority of cases, the standard test that will be offered at an HIV
testing centre will be an HIV antibody test. The HIV antibody test looks for antibodies
to the virus in a person's blood. For most people these antibodies take 6 weeks to 3 months to develop. In very rare cases, it can take up to 6 months.
Getting tested before 3 months have elapsed may result in an unclear test result, as an infected person may not yet have developed antibodies to HIV. These 3 months are known as the 'window period'. So it is best to wait for at least 3 months after the last time you were at risk before taking the test. Some test centres may recommend testing again at 6 months, just to be extra sure, though in most cases this is not necessary.
If you are waiting to take an HIV antibody test, it is vital that you do not put yourself at risk through further exposures to HIV during this time period. This means you should practice safe sex
and not share needles. Any further exposure to HIV may mean a negative HIV test result is not accurate (unless of course you wait another three months after this exposure).